1. The Field of the Invention
The present invention relates generally to x-ray tubes. More particularly, embodiments of the present invention relate to an x-ray tube cathode that integrates several x-ray tube components into a single unified assembly so as to significantly improve cathode efficiency and electron beam generations, and thereby, the overall performance of the device.
2. The Prior State of the Art
X-ray producing devices are extremely valuable tools that are used in a wide variety of applications, both industrial and medical. For example, such equipment is commonly used in areas such as diagnostic and therapeutic radiology; semiconductor manufacture and fabrication; and materials analysis and testing. While used in a number of different applications, the basic operation of x-ray tubes is similar. In general, x-rays, or x-ray radiation, are produced when electrons are produced, accelerated, and then impinged upon a material of a particular composition.
Typically, this process is carried out within an evacuated enclosure, or “can.” Disposed within the can is an electron generator, or cathode, and a target anode, which is spaced apart from the cathode. In operation, electrical power is applied to a filament portion of the cathode, which causes electrons to be emitted. A high voltage potential is then placed between the anode and the cathode, which causes the emitted electrons accelerate towards a target surface positioned on the anode. Typically, the electrons are “focused” into a primary electron beam towards a desired “focal spot” located at the target surface. In addition, some x-ray tubes employ a deflector device to control the direction of the primary electron beam. For example, a deflector device can be a magnetic coil disposed around an aperture that is disposed between the cathode and the target anode. The magnetic coil is used to produce a magnetic field that alters the direction of the primary electron beam. The magnetic force can thus be used to manipulate the direction of the beam, and thereby adjust the position of the focal spot on the anode target surface. A deflection device can be used to control the size and/or shape of the focal spot.
During operation of an x-ray tube, the electrons in the primary electron beam strike the target anode surface (or focal track) at a high velocity. The target surface on the target anode is composed of a material having a high atomic number, and a portion of the kinetic energy of the striking electron stream is thus converted to electromagnetic waves of very high frequency, i.e., x-rays. The resulting x-rays emanate from the target surface, and are then collimated through a window formed in the x-ray tube for penetration into an object, such as a patient's body. As is well known, the x-rays can be used for therapeutic treatment, or for x-ray medical diagnostic examination or material analysis procedures.
As suggested above, the typical x-ray tube includes a filament portion, or emitter, that emits electrons by the process of thermionic emission. In particular, it is a characteristic of the emitter that, when heated, as by the passage of an electrical current therethrough, it emits a cloud of electrons. The emitted electrons, in turn, are focused into a beam of a desired diameter, directed at the target surface of the target anode. In the cathodes of known x-ray devices, the focusing process involves substantially enclosing the emitter with a structure defining an opening, or focusing slot, having a desired geometry, so as to allow only a portion of the emitted electrons through the focusing slot. In typical x-ray tubes, the electron emission and focusing functions are performed by an assembly comprising as many as eleven different parts.
For example, some known x-ray tubes typically employ, in addition to the emitter, means for isolating the emitter, as well as a structure for attaching the emitter to the cathode. In similar fashion, the emitted electrons are typically focused into a beam by an assembly that includes at least a focusing cup, focusing slots, and focusing tabs.
The large number of parts in the assemblies typically employed to perform the emission and focusing functions of the x-ray tube produces a variety of undesirable consequences. For example, the cost of such assemblies is necessarily higher than it would otherwise be in view of the large number of small parts that must be separately manufactured. It likewise follows that assembly costs for such devices are correspondingly higher, in view of the large number of parts comprising the assembly and the numerous operations required to assemble those parts.
Another problem with the use of assemblies employing multiple parts relates to the inevitable inaccuracies and errors that result during production of those parts. As is well known, various parameters of manufactured parts are allowed to vary within a permissible range. This range is typically referred to as the “tolerance” for that part. Electron emitting and focusing assemblies comprising multiple parts, each with its own range of tolerances, are problematic because while the parameters of a single part may be within an acceptable range, the cumulative effect of assembling a variety of parts, each of whose tolerances is allowed to vary, is that the integrity and/or performance of the x-ray device as a whole may be significantly compromised.
Furthermore, the use of multiple parts in assembling the emission and focusing structures of the typical x-ray tube greatly increases the opportunity for part combinations to fail either during manufacture or during operation of the x-ray device. That is, each connection between parts represents a potential failure point for the device.
Other significant problems in known x-ray tubes concern the characteristics of particular emitters, the geometry of the emitter, and the geometry of the components used to focus the emitted electrons, and the implications that those various geometries have for the overall performance of the x-ray device.
As discussed elsewhere herein, x-ray devices employ emitters that discharge electrons by a process generally known as thermionic emission. Each emitter has a characteristic often referred to as its “perveance.” Specifically, the perveance of a particular emitter is related to the number of electrons discharged by an emitter and received at a target anode disposed a given distance away from the emitter. In general, a given target anode receives relatively more electrons from an emitter having a relatively higher perveance than from an emitter with a relatively lower perveance, i.e., the perveance value of a given emitter is proportional to the number of electrons discharged by that emitter and received at the target anode.
It is generally acknowledged that diagnostic image quality is at least partially a function of the number of electrons that impinge upon the target surface of the target anode, so that, in general, the more electrons that reach the target surface, the better the resulting image. The performance of a particular emitter can thus be evaluated in terms of the efficiency of that emitter, where the efficiency of the emitter is defined as the number of electrons impinging upon the target surface of the target anode, i.e., the perveance of the emitter, as a percentage of the total number of electrons discharged by the emitter. In general then, image quality improves as the efficiency of the emitter increases.
While the quality of the images generated by an x-ray device is to a large extent a function of emitter efficiency, it is also well understood that the quality of the diagnostic images additionally depends on the pattern, or focal spot, created by the emitted electrons on the target surface of the target anode. In general, smaller focal spots tend to produce better quality images than do larger, more diffuse focal spots.
In view of the foregoing principles, a variety of attempts have been made to improve emitter efficiency and to concentrate the electrons discharged from the emitter so that the electron beam thus formed is highly focused at the point where it impacts the target anode. As discussed in further detail below however, emitter efficiency and focal spot size are closely related, and success in improving one has typically been achieved only at the expense of the other.
In general, attempts to concentrate emitted electrons into a focused beam have placed emphasis on development of various geometries designed to enclose a portion of the emitter so that electrons that are free to leave the emitter do so in a defined pattern. The configuration typically employed in known x-ray tubes generally includes a long, slender emitter made of tungsten or similar material, substantially enclosed by a rectangular or box-shaped focusing assembly that defines a small opening, or focusing slot. While a rod-shaped emitter discharges uniform numbers of electrons radially in all directions, only those electrons that are able to pass through the focusing slot reach the target surface of the target anode. That is, the shapes of the emitter and focusing slot are not complementary, but rather are arranged so that the direction of travel, or velocity vectors, of the majority of the emitted electrons is generally not in the primary beam direction. Such arrangements, while producing a relatively focused beam of electrons, are nevertheless inefficient in that relatively few of the emitted electrons impinge upon the target surface of the target anode. As previously noted, diagnostic image quality is compromised by inefficient emitters.
Accordingly, the focusing slot must be sufficiently large to pass enough electrons to achieve a desirable emitter efficiency. As discussed below however, increasing the size of the focusing slot introduces at least one significant problem.
As noted earlier, the emitters typically employed in know x-ray devices tend to discharge a large number of electrons whose velocity vectors are not in the desired direction of the electron beam. Rather, many of these electrons travel only in the general direction of the target surface of the target anode, along paths that are divergent from the primary beam direction. As a result, the pattern defined on the target surface of the target anode, i.e., the focal spot, is larger than it would be if the majority of the electrons traveled in the primary beam direction. Thus, while relatively larger focusing slots facilitate some improvement in emitter efficiency, they also result in larger focal spots which compromise the quality of the diagnostic images produced by the x-ray device.
Those skilled in the art are aware of the tension between focal spot size and emitter efficiency. As a result, at least one attempt has been made to resolve the problem. However, as discussed below, this attempted resolution fails to adequately address the problems enumerated herein.
In particular, a focusing element has been developed that does not substantially enclose the emitter, but rather assumes the shape of the high voltage field contours present in the x-ray device in an attempt to direct emitted electrons in a narrow beam towards the target surface of the target anode. While such a focusing element arguably improves emission efficiency by allowing more electrons to reach the target surface of the target anode, the focal spot produced by the emitted electrons becomes larger and consequently more diffuse, thereby compromising the quality of the images produced by the device.
Additionally, because the emitter and the focusing element have different electrical continuities, an irregularity is typically formed in the high voltage field contours. As a result of the irregularity in the high voltage field contours, a significant portion of the electrons discharged by the emitter diverge from the primary beam direction. Consequently, the overall diameter of the electron beam produced by this arrangement is relatively larger than would otherwise be the case, and thereby results in a correspondingly larger focal spot on the target anode. Such a result further exacerbates the focal spot problems imposed by the geometry of this focusing element. Finally, the complex shape of such focusing elements makes them difficult to machine, and therefore, very expensive.
Not only are known emitter and focusing element geometries inherently limited in terms of their ability to produce optimum focal spots, but the inadequacies of those geometries are further aggravated by changes that can occur in the spacing between the anode and the cathode. In particular, because those geometries tend to produce a relatively more diffuse electron beam, any change in the spacing between the anode and the cathode tends to exacerbate that effect and thereby causes the beam to become more diffuse. As discussed elsewhere herein, such diffuse beams produce large focal spots that are not conducive to high quality images. Because the distance between the anode and the cathode may vary during operation of the x-ray device, the sensitivity of known emitter and focusing element geometries to such variations is a significant limitation.
Finally, at least one other limitation imposed by known emitter and focusing element geometries concerns changes in the beam current of the device. In general, “beam current” refers to the amount of current flow, or the number of electrons, traveling from the emitter to the anode. Changes in the beam current, such as may be required for various different types of exposures, tend to increase or decrease the size of the focal spot produced by the beam. For example, a relative increase in beam current increases the size of the focal spot produced by the beam. The phenomenon is particularly problematic where, as in the case of typical x-ray devices, the emitter and focusing element arrangement is such that many of the electrons in the electron beam travel along paths divergent from the primary beam direction and thus tend to contribute to relatively larger focal spots.
As discussed elsewhere herein, a large focal spot is undesirable. However, while a reduction in beam current would produce a smaller focal spot, a relatively lower level of beam current may not be appropriate or adequate in some applications. Thus, in known x-ray devices, the size of the focal spot is highly sensitive to changes in beam current. Such changes in beam current are commonly known as “blooming.” Blooming is undesirable because it tends to compromise the quality of the images produced by the device and/or it compromises the flexibility of the device.
In addition to the shortcomings of known focusing element and emitter geometries, the cathode support structures typically employed in x-ray devices are problematic as well. In particular, known cathode support structures are problematic at least because they employ a large number of separate parts that must be separately manufactured and assembled. The use of a large number of parts necessarily implicates relatively higher assembly and production costs than would otherwise be the case. For example, a typical cathode support structure includes such components as filament lead ceramics, a cathode cup, a filament lead cathode cup mounting arrangement, and the filament-to-cup attachment mechanism. As suggested elsewhere, such a multiplicity of parts, in addition to imposing relatively higher manufacturing costs, also introduces numerous potential failure points in the x-ray device.
In view of the foregoing problems and shortcomings with the existing x-ray tube cathodes, it would be an advancement in the art to provide a cathode, and associated cathode support structure, that is simple and relatively inexpensive to manufacture. Also, the cathode should be highly efficient in terms of electron emission and should produce a focal spot that is substantially insensitive to changes in operating conditions such as anode-to-cathode spacing, or variations in beam current.